The present invention relates to a torsional vibration measuring instrument and a torsional vibration measuring method especially for measuring the fluctuation of displacement of rotational angle of a rotary shaft in a diesel engine and the like for shipboard or electric power generation.
A rotary shaft of a shipboard large diesel engine or the like for transmitting power to a load is subject to torsional vibration caused by intermittent explosion in a piston or pistons of the engine, thereby producing resonance with natural frequency according to the length of the rotary shaft. From standpoints of safety and basic performance capacities, the torsional vibration must be detected for preventing the engine from such resonance and noise. Conventionally, there has been used an instrument for measuring such torsional vibration.
Referring to FIG. 11 showing the system of the conventional analog torsional vibration measuring instrument (disclosed in Japanese Patent No. Hei 1-27067), a load 5 is supplied with the rotational driving power from a driving source 1 like an engine through a rotary shaft 2.
A gear 3, which has a determined number of teeth, is fixed onto a certain portion of rotary shaft 2. Gear 3 and a pickup 4, e.g. electromagnetically operated, which is opposed to gear 3, constitute a rotation detector (hereinafter, an encoder).
Pickup 4 detects rotation pulses every rotation of rotary shaft 2 at a predetermined angle xcex8, and transmits the detected pulses to an input terminal T1 of a torsion vibration measuring instrument 6.
Rotation pulses, through input terminal T1, are amplified by an amplifier 7, and are transmitted to a phase detecting circuit 9 of a phase lock loop (PLL) 8, so as to measure the phase displacement between the pulses from pickup 4 and those generated by a voltage control oscillating (VCO) circuit 11.
The detected phase displacement signal is smoothed through a low pass filter (LPF) 10. The signal filtrated by LPF 10 is changed by VCO 11 into a voltage corresponding to a natural frequency in proportion to the rotational speed of rotary shaft 2 without torsional vibration. VCO 11 outputs reference pulses with a frequency corresponding to the voltage, so as to transmit them to both phase detecting circuit 9 and a phase displacement measuring circuit 12.
Phase displacement measuring circuit 12 also receives rotation pulses to be measured from the encoder through amplifier 7.
As disclosed in Japanese Patent No. Sho 57-6052, phase displacement measuring circuit 12 is configured to calculate the digital value of displacement between both the inputs every input period, i.e., the phase displacement between the reference pulse from VCO 11 and the rotation pulse to be measured from the encoder, which corresponds to the angular fluctuation generated by the torsional vibration of rotary shaft 2, so as to transmit it to an output terminal T3. Output terminal T3 is connected through a cord or the like with a chart recorder 13, an FFT (a fast Fourier transform) analyzer 14 or the like for recording and analysis of a torsion angle in relation to a crank angle or rotational angle.
The torsional vibration measuring instrument disclosed in Japanese Laid Open Gazette No. Hei 6-307922, as shown in FIG. 12, is perfectly digitized so as to free its measuring range from the capacities of circuits. In FIG. 12, the components coinciding with those in FIG. 11 are marked by the same reference numerals without description. Furthermore, a striped tape 3a is stuck to the periphery of rotary shaft 2, and a pair of photosensors 4a and 4b such as photodiodes are disposed at a reference angle xcex8 from a surface which is perpendicular to the longitudinal direction of rotary shaft 2 and opposite to tape 3a. 
The rotation pulses output from photosensors 4a and 4b pass through input terminals T1 and T2 and first and second amplifiers 7a and 7b, respectively, so as to be inputted into a digital processing circuit 15 in torsional vibration measuring instrument 6. Digital processing circuit 15 comprises a period calculating unit, a sample period measuring unit, a sum averaging unit, a rotation angle displacement fluctuation calculating unit and the like, so as to send a calculated signal xcex94xcex8 of fluctuation of rotation angle displacement of rotary shaft 2 to first output terminal T3. Various pulse shapes are analyzed from fluctuation signal xcex94xcex8 by FFT analyzer 14 or the like which is connected with first output terminal T3. Furthermore, fluctuation signal xcex94xcex8 is sent to a second output terminal T4 through a digital-analog convertor (DAC) 16, so as to be recorded by recorder 13 connected to second output terminal T4. This torsional vibration measuring instrument can reliably detect the seam on tape 3a by timers of photosensors 4a and 4b. 
The above-said conventional digitalized torsional vibration measuring instrument has involved the problem that a distance between opposed photosensors 4a and 4b above tape 3a must be smaller than the pitch of each stripe on tape 3a, thereby requiring a long time for the burdensome arrangement of photosensors 4a and 4b and making the instrument expensive.
Furthermore, the same instrument has been of an installed type requiring a large casing. When the torsional vibration measurement is to be performed at a worksite in a factory or the like, recorder 13 and FFT analyzer 14 must be carried together with such a large instrument.
The present invention provides a torsional vibration measuring instrument and a torsional vibration measuring method for solving the above problems. A first object of the invention is to obtain a miniaturized portable torsional vibration measuring instrument, which can be placed in a narrow space of a ship, a power plant or the like, memorize the torsional vibration data fast with reliability and display the memorized data, and to obtain a method for such measurement by use of the instrument.
A second object of the invention is to obtain a torsional vibration measuring instrument including a striped tape stuck onto a rotary shaft and a torsional vibration measuring method, wherein, when the rotational period between juxtaposed stripes of the tape across a seam thereon is different from that between regularly juxtaposed stripes thereof (when the fluctuation of rotational angle displacement between the juxtaposed stripes across the seam is large), the different period across the seam is replaced with the average of the forward and rearward periods, thereby avoiding difficult adjustment of the seam and the detection jump across the seam.
The torsional vibration measuring instrument of the present invention comprises a detection means for detecting a period every a predetermined angle during one rotation of a rotated object, a storage means for recording the period detected by the detection means, an average calculation means for calculating an average of the detected period, a torsional angle calculation means for calculating a torsional angle of the rotated object from the average calculated by the average calculation means and from the period detected by the detection means, and a display means for displaying at least the amplitude of torsional angle.
The torsional vibration measuring method of the present invention is to remove the storage means which has recorded the value detected by the detection means, from the portable torsional vibration measuring instrument with the display means, and attach the storage means to a data analyzing/processing means, thereby enabling the data analyzing/processing means to print out the analyzed data or communicate the data signal.